Feline Heartworm Disease
Clarke E. Atkins, D.V.M.,
Diplomate, A.C.V.I.M (Internal Medicine and Cardiology)
William G. Ryan, M.B.A., B.V.Sc.
Infection of a cat with Dirofilaria immitis was
first recognized in the United States in 1922. There have been numerous subsequent
reports, with a recent review of 156 previously reported cases. Despite this, feline
heartworm disease (FHWD) has generally been considered a novelty, with many veterinarians
still believing that the cat is not at risk. This is an unfortunate misconception, as not
only is the cat susceptible, but also its clinical signs are more severe than those of the
dog, even when the worm burden is quite small. In recent years, increasing awareness has
brought the development of better diagnostic tests, efforts at establishing a satisfactory
adulticidal therapy, increasing public awareness, and, recently, the registration of a
feline heartworm preventive.
A review of studies reporting feline heartworm infection (FHWI) and a survey of veterinary practitioners revealed that the diagnosis has been made in 38 of the 50 United States. Not surprisingly, the greatest numbers of cases have been reported from the southeastern United States, the Eastern Seaboard, and the Gulf Coast and within the Mississippi River valley. Prevalence studies have focused mainly on cats from shelters. While this population choice has allowed the use of the relatively sensitive and very specific postmortem diagnosis, these studies are not necessarily applicable to pet cats, even in the same geographic region. These 12 studies, from 10 southeastern states, have revealed a prevalence of FHWI ranging from 0 to 16%. There are limited data in pet cats, but a joint study, performed on such cats presented to the teaching hospitals of North Carolina State University (NCSU) and Texas A&M University for evaluation of cardiorespiratory signs, demonstrated an infection prevalence of 9% and an exposure rate of 26%, the latter figure based on antibody titers. A number of serologic surveys, largely in asymptomatic cats, have demonstrated exposure rates (antibody seropositivity) of 5 to 36%, even in areas not considered heavily heartworm endemic and as high as 43% in asymptomatic cats.
The diagnosis of FHW1 or FHWD poses a unique and problematic set of
issues. First, the clinical signs in cats are often
quite different from those in dogs. Then, the diagnostic effort is often inadequate because the suspected incidence in cats is
low. Furthermore, the diagnosis is often elusive because eosinophilia is transient or absent; electrocardiographic findings are
minimal; and most cats are amicrofilaremic. Radiography, while helpful, is neither adequately sensitive nor specific and
requires expertise in interpretation. Echocardiography shows promise in terms of specificity but is costly and only moderately sensitive, and requires special equipment and expertise. Currently, the most useful tests include ELISA serologic tests. These too are imperfect. The antigen test is very specific but is inadequately sensitive, missing over 50% of natural infections. On the other hand, a recently developed feline antibody test is very sensitive, but its specificity is low, meaning that a positive test indicates -exposure, but not necessarily adult infection.
While no breeds of cat have been shown to be a increased risk for
FHWI, most authors do suspect a male predisposition.
This suspicion is based on the overall preponderance of male cats diagnosed with FHWI and the greater experimental infection rate in males than in females. The experience at NCSU suggests, however, that while more males (61%) than females are indeed diagnosed with FHWI, the male-to-female ratio is not significantly different from that of the general population of cats seen at a teaching hospital or the population of cats presented with cardiorespiratory signs;. The typical cat with FHWI is 4 to
6 years of age (range <1 to 19 years). The history of outdoor exposure would logically predict a heightened risk of FHWI;
the experience at NCSU indicates that this is true. Nevertheless, one third of heartworm-infected cats are reported by their owners to be housed totally indoors. This may mean that owners misinterpret the question as to whether their pet goes outdoors or that indoor cats can be infected, or both. Although a seasonal incidence (August to December) has been suggested for FHWD, other studies do not support this contention.
Heartworm-infected cats may be
asymptomatic, and clinical manifestations, when present, may take either an acute (often
cataclysmic) or a chronic (often waxing and waning) course. Acute or peracute presentation
is usually due to dead worm embolization or migration of worms to the central nervous
system, and signs variably include salivation, tachycardia, shock, dyspnea, cough or
hemoptysis, vomiting and diarrhea, syncope, dementia, ataxia, circling, head tilt,
blindness, seizures, and death. Sudden death, with little or no premonitory signs, has
been observed in approximately 23% of cases. Cats that die suddenly may appear clinically normal up to 1 hour before death.
Postmortem examination has revealed as few as two worms in cats that have died
suddenly. Postmortem examination
typically reveals pulmonary infarction with congestion and edema. Vena caval syndrome has
also been recognized in cats.
Findings in chronic FHWD may include cough, dyspnea, anorexia, weight loss, lethargy, exercise intolerance, vomiting, and signs of right-heart failure. Cough is a relatively consistent finding (>50% of cases, compared with 15% in cats with cardiorespiratory signs, but not FHWI and, when noted in cats in endemic areas, should increase the suspicion of FHWD. Likewise, dyspnea, though less specific than cough, is present in 40 to 60% of cases. The pulmonary response to in situ heartworms in cats includes type 11 cell hyperplasia and activation of pulmonary intravascular macrophages. This response, not recognized in dogs, may explain the astlima-like syndrome recognized in some cats, even after they have been cleared of D. immitis.
Physical examination is often unrewarding, although a murmur, gallop, and/or diminished or adventitial lung sounds may be noted. In addition, cats may be thin and/or dyspneic. If heart failure is present, jugular venous distention, pleural effusion, and rarely ascites are detected
Hematology. Although the presence of eosinophilia
or basophilia may increase the index of suspicion for FHWL tests for these conditions are
of limited value. This is true because these hematologic changes are transient (present at
4 to 7 months after infection) and present in only 33% of cases. In a prospective study of
cats with cardiorespiratory signs, those with FHWD were not significantly more apt to have
eosinophilia or basophilia than those not shown to be infected.
Microfilarial Tests. A definitive diagnosis of FHWl can be made by the detection of circulating microfilariae, using the modified Knott test, millipore filter, direct smear, or microhematocrit techniques. A recent literature review indicated that 36% of 45 cats with FHWD were microfilaremic, while other reports have indicated no more than 20% of infected cats are microfilaremic. This discrepancy probably reflects the fact that at the time of early reports diagnostic methods were limited to microfilarial tests and postmortem examination. While increasing the volume of blood samples, multiple testing, and drawing evening samples may increase the diagnostic efficiency of the microfilariadependent tests, the low percentage of cats that become microfilaremic, the transient nature of microfilaremia, and the low microfilarial numbers seriously limit their utility.
Radiography. Radiographic findings of FHWD include enlarged caudal pulmonary arteries, often with ill-defined margins; pulmonary parenchymal changes, including focal or diffuse infiltrates (interstitial, bronchointerstitial, or even alveolar); perivascular density; and, occasionally, atelectasis or pleural effusion. Pulmonary hyperinflation and right-heart enlargement may also be evident. Thoracic radiography has been suggested as an excellent screening test for FHWD. However, while often helpful, thoracic radiography is neither sensitive nor specific in making the diagnosis of FHWD. The single most sensitive radiographic criterion (left caudal pulmonary artery diameter greater than 1.6 times that of the ninth rib at the ninth intercostal space) can be identified in only 53% of cases and may also be noted in cats with heart failure, but not FHWI. Likewise, pulmonary parenchymal changes are only detectable radiographically in approximately 50% of natural cases. Even though most cats with clinical signs have some radiographic abnormality, the findings are not specific for FHWD, are variable, and are often transient. Lastly, radiographic abnormalities have been detected in experimentally exposed cats that ultimately resisted heartworm maturation and were negative on postmortem examination (i.e., false-positive). On the other hand, pulmonary angiography can be used to make a definitive diagnosis by the demonstration of radiolucent intravascular "foreign bodies," as well as enlarged, tortuous, and blunted pulmonary arteries.
A,Thoracic radiograph from a cat with
mild radiographic signs of heartworm disease. Note the right caudal pulmonary artery
(arrow). The arrow is located in the ninth intercostal space, the site for comparison of
the ninth rib with the pulmonary artery (right or left side). If the pulmonary artery is
greater than 1.6 times the size of the rib, it is suggestive of heartworm disease (Schafer
and Berry, 1995). B, Thoracic radiograph obtained from a more severely affected cat. Note
the alveolar infiltrate in the caudal lung lobes. (From Schafer M, Berry CR: Cardiac and
pulmonary artery mensuration in feline heartworm disease. Vet Radiol Ultrasound 36:499,
Echocardiography. Echocardiography is more
sensitive in cats than in dogs for the detection of heartworm infection. Typically, a
"double-lined echodensity" (Fig. 4) is evident in the main pulmonary artery, one
of its branches, or the right ventricle, or occasionally at the right atrioventricular
junction. FHWI was detected echocardiographically in 7 of 9 natural cases and 12 of 16
experimental infections. A retrospective review of a larger case series revealed a lower
sensitivity when worms were not specifically sought and, particularly, when studies were
performed by noncardiologists. This observation underscores the need for a high index of
suspicion and expertise if this technique is to be of value in the diagnosis of FHWI.
Ivermectin, administered as a single dose of
24 jig/kg, and milbernycin oxime, administered at 500 mcg/kg in two doses 1 month apart,
effectively prevent FHWI when exposure has occurred during the previous month. In late
1996, the FDA approved ivermectin as a feline heartworm preventive for administration in a
chewable formulation (Heartgard for Cats, Merial, Iselin, NJ). Ivermectin has been
demonstrated to have an excellent safety profile, with no adverse effects in kittens at
doses exceeding 110 mcg/kg and doses of at least 750 mcg/kg in adults. Although not
marketed for cats, milbernycin has recently received FDA approval as a feline heartworm
preventive at a dosage of 2000 mcg/kg.
Even though there is no reason to expect adverse reactions to prophylaxis in cats with existing FHWI, it could be useful to know the heartworm status of cats prior to the administration of a preventive. The current ELISA antigen tests have not yet been shown to be adequately sensitive for this purpose, unless the client is properly educated as to the limitations of the test. The ELISA antibody test (alone or with an antigen test) is currently more appropriate because of its higher sensitivity and ability to identify cats at risk (infected or exposed). While screening for FHWI before the administration of a preventive is not absolutely necessary, client education as to the possibility of preexisting infection is imperative.
Based on disease severity, the lack of an effective and safe
adulticidal therapy, and the difficulty in making a definitive diagnosis, the authors
believe that cat owners in endemic areas should be offered the choice of their pet
receiving a heartworm preventive. Clearly, cats already infected with heartworms and their
house mates should be placed on a
Atkins CE, DeFrancesco TD, Miller M, et al: Prevalence of heartworm
infection in cats with signs of cardiorespiratory abnormalities.
J Vet Med Assoc 212:517, 1997.
A prospective survey of the prevalence of heartworm infection in North Carolina and Texas cats with cardiorespiratory abnormalities. Prevalence, risk factors, and comparisons of diagnostic tests are provided.
Atkins CE: Veterinary CE Advisor: Heartworm disease: An update. Vet
Med (Suppl) 93:12:2, 1998a.
A comprehensive and very current review of the diagnosis and preven tion of heartworm infection in dogs and cats.
Atkins CE, DeFrancesco TD, Coats J, et al: Feline Heartworm Disease
The North Carolina Experience. In: Soll MD, Knight DH, eds:
Proceedings of the American Heartworm Symposium '98. Batavia, IL American Heartworm Society, 1998b.
Retrospective analysis of the risk factors, clinical presentation,
diag nostic test results, and survival of 50 cats with heartworm infection.
Clark JN, Pulliam JD, Alva R, et al: Safety of orally administered ivermectin in cats. In: Soll MD, ed: Proceedings of the Heartworm Symposium '92. Batavia, IL: American Heartworm Society, 1992, p 103.
Documents the safety of high-dose ivermectin in adult cats.
DeFrancesco TD, Atkins CE, Miller MW, et al: Diagnostic utility of
echocardiography in feline heartworm disease. In: Soll MD, Knight DH,
eds: Proceedings of the American Heartworm Symposium '98.. Batavia, IL: American Heartworm Society, 1998.
A review of a number of reports of the efficacy of echocardiography in the diagnosis offeline heartworm infection.
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